Blizzard has announced on both their Customer Service Twitter and Battle.net Client that their authentication services are currently under a DDoS attack, which may lead to players experiencing Battle.net disconnects and slow login attempts.
What is a DDoS Attack?From Wikipedia:
Distributed DoS A distributed denial-of-service (DDoS) attack occurs when multiple systems flood the bandwidth or resources of a targeted system, usually one or more web servers. A DDoS attack uses more than one Read more on wowhead.com